How does a cookie work across domains?

How does one enable cross-origin cookie validity? Why? Recently, this problem has been quite depressing ., /, Document. domain. this is the COOKIE path. Instead, it should be the COOKIE domain. how does the cookie cross-origin function work?
Http://blog.xxx.com/and http://www.xxx.com/blog/

How can I make cookies valid?
Recently, it has been quite depressing.

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/
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Document. domain
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This is the COOKIE path, but the COOKIE domain.
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Use a database to simulate cookie storage. This seems a little better.

Other methods ...... A little hard?
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Add domain name:
'.Xxx.com'